Various processes for the fabrication of waveguide couplers are known. For example, U.S. Pat. No. 3,579,316, and International Publication No. WO87/00934 published under the Patent Cooperation Treaty disclose such fabrication processes. Other waveguide coupler fabrication techniques are disclosed in, e.g., N. TAKATO al: "LOW-LOSS HIGH-SILICA SINGLE-MODE CHANNEL WAVEGUIDES", Electronics Letters Vol.22 No.6 pp.321-322, BUCHMANN et al: "REACTIVE ION ETCHED GaAs OPTICAL WAVEGUIDE MODULATORS WITH LOW LOSS AND HIGH SPEED", ibid., Vol.20 No.7 pp.295-297. A conventional optical waveguide coupler such as disclosed in the above prior references uses the same material for waveguide portions. However, the drawback of the conventional optical waveguide coupler is that the splitting ratio is greatly dependent upon the coupler length. It is known that the splitting ratio is essentially defined by a function of the coupling coefficient, the propagation constant of waveguide portions and the coupling length. As shown in FIG. 1, light entering into an incident port 1 of an optical fiber couples into a closely placed core in another optical fiber. The splitting ratio R is given by a logarithm of the ratio of light intensities in cores a and b, Pb/Pa, as follows: ##EQU1## where X is the coupling coefficient, .DELTA. is a half of the propagation constant difference, Z is a coupling length from the start of coupling and .beta.c=.sqroot.X.sup.2 +.DELTA..sup.2. Conventionally, closely placed waveguides are made of the same material and, therefore the propagation constant difference .DELTA.=0 and .beta.c=X. Thus the splitting ratio is given as R=10 log Tan.sup.2 XZ. Therefore as shown in FIG. 2, the splitting ratio is governed by a periodic function of the coupling length Z in which complete power transfer from the core a to core b takes place at distances of integer multiples of .pi./2X.
Recently, there are growing demands in measurements and optical communications to monitor signal conditions in main communication lines without disturbing the information in the main lines. The use of optical waveguide couplers is easy and inexpensive to accomplish this objective. For such monitoring purpose, the required splitting ratio R is selected to be 20 to 40 dB. If the same material is used, a slight difference in coupling length causes a large change in the splitting ratio R as seen in FIG. 2. Therefore, the control of coupling length to obtain a desired splitting ratio during manufacture is very difficult, which consequently results in many splitting ratio deviations. In addition, there is another problem in which a slight change in external condition such as temperature causes a slight change in coupling length thereby resulting in a large change in the splitting ratio.